A trio of retired detectives, Kevin Gannon, Anthony Duarte and Mike "Mikey" Donovan, claim that the deaths of hundreds of men across the U.S., labeled as "accidental drownings," are actually murders, the work of more than a single killer.

"You're talking about almost 300 young men that have been drugged, abducted, held for a period of time, murdered on land and deposited in body of water to make it look like a drowning," Gannon told Inside Edition.

They say the deaths are all linked by a single chilling clue: graffiti of a smiley face found near the bodies. The detectives have dubbed the alleged ring the "Smiley Face Killers," and they say it's made up of both women and men.

"We've been tracking [them] for 12 years now," said Gannon. "... This is a well-structured organized group with cells — spread out in major cities across the U.S."

The theory is laid out in the riveting Oxygen docu-series "Smiley Face Killers: The Hunt for Justice."

Stephany Welzien's son Brian, a Navy veteran, was just 21 when he disappeared on New Year's Day in 2000. His remains weren't found until two months later, floating in a lake in Gary, Indiana.

"He was just a kid next door that you'd want your daughter to date," Welzien told Inside Edition.

"Each of our victims fit such a narrowly defined specific set of demographics," Duarte explained.

In 2009, 24-year-old William Hurley's death was eerily similar to that of Brian. He was found in the Charles River in Boston six days after he was last seen. Nearby? A smiley face.

Could these men, and hundreds of others, have fallen victim to the "Smiley Face Killers"?

The FBI says not likely. "We have not developed any evidence substantiating the theory that these deaths are the work of a serial killer or killers," the agency said in a statement. "The vast majority of these instances appear to be alcohol-related drownings."

But the mothers who Inside Edition spoke to remain convinced their sons' deaths were not accidental.